1. Field of the Invention
This invention relates to optical systems and, more particularly, to an optical system used for laser beam printers, digital PPC and the like which has a semiconductor laser accommodated within a case to emit laser light to be delivered to a collimator lens through a window formed in the case.
2. Description of the Prior Art
There is shown, in FIG. 6, a laser beam printer 1 having a conventional optical system of this kind. In this laser beam printer 1, a semiconductor laser unit 2 emits laser light to be irradiated to a photosensitive drum 8 through a collimator lens 3, a cylinder lens 4, a polygon mirror 5, f-.theta. lenses 6a and 6b, a reflection mirror 7 and so on. Due to this, an electrostatic latent image is formed on the photosensitive drum 8, which is then toner-developed so that a toner image formed is transferred onto a recording paper.
The semiconductor laser unit 2 used in this laser beam printer 1 includes, as shown in FIG. 7, a case 2b having a window 2a. Within the case 2b, a heat-sink 2c is provided. The heat-sink 2c has a semiconductor laser 2e mounted on a lateral surface thereof through a sub-mount 2d. A cover formed of a transparent material, such as glass, is fitted on an inner side of the window 2a. Meanwhile, the collimator lens 3 is arranged at one end of a lens barrel 3a, while the lens barrel 3a at the other end is faced to the window 2a of the semiconductor laser unit 2. Consequently, the laser light emitted from the semiconductor laser 2e is given onto the collimator lens 3 through the window 2a and lens barrel 3a, and further to a polygon mirror 5 via the cylindrical lens 4.
In the prior art, the laser light emitted from the semiconductor laser 2e is delivered to the collimator lens 3 through the window 2a and lens barrel 3a. Accordingly, part of the laser light reflects upon the window 2a or the lens barrel 3a and thus turns into stray light. Such stray light might travel off the normal path and go onto the photosensitive drum 8. For example, if part of laser light deviates from the polygon mirror 5 and comes to and reflected upon a drive motor 5a or the like, there is a fear that the laser light acts as a rest beam on the photosensitive drum 8 thus resulting in undesired printing (vertical straight line).